1. Field
This document relates to a 3D image display device and a driving method thereof for improving picture quality.
2. Related Art
With the advancement of various image processing techniques, 3D image display systems capable of displaying 3D images are developed.
Methods of generating 3D images are divided into a stereoscopic technique and an autostereoscopic technique.
The stereoscopic technique uses disparity images of left and right eyes, which have high 3D effect, and includes a stereoscopic method and an autostereoscopic method which are practically used. The autostereoscopic method provides an optical plate such as a parallax barrier for separating optical axes of left and right disparity images from each other before or behind a display screen. The stereoscopic method displays left and right disparity images having different polarization directions on a liquid crystal display panel and generates 3D images by using polarizing glasses or liquid crystal shutter glasses.
The stereoscopic method is divided into a first polarizing filter method using a pattern retarder film and polarizing glasses, a second polarizing filter method using a switching liquid crystal layer and polarizing glasses, and a liquid crystal shutter glasses method. In the first and second polarizing filter methods, 3D images have low transmissivity due to the pattern retarder film or the switching liquid crystal layer, which is arranged on a liquid crystal display panel to function as a polarizing filter.
The liquid crystal shutter glasses method alternately displays left-eye and right-eye images on a display frame by frame and opens/closes left-eye and right-eye shutters of liquid crystal shutter glasses in synchronization with the display timing to generate a 3D image. The liquid crystal shutter glasses open only the left-eye shutter for an nth frame period in which a left-eye image is displayed and open only the right-eye shutter for an (n+1)th frame period in which a right-eye image is displayed to generate binocular disparity in a time division manner.
In 3D image display devices, a liquid crystal display (LCD) is widely used as an image display device. The LCD, a hold type display device, holds data charged in a previous frame right before new data is written because of maintenance characteristic of liquid crystal. The response of liquid crystal is delayed according to data writing. The response delay of liquid crystal causes motion blurring when a left-eye image is changed to a right-eye image or when a right-eye image is changed to a left-eye image while the LCD generates a 3D image to result in 3D crosstalk in the form of a ghost.
Various methods for improving the response characteristic of liquid crystal for 2D images are known. Over driving control (ODC) modulation compares previous frame data and current frame data to each other, detects a data variation according to the comparison result, reads a compensation value corresponding to the data variation from a memory and modulates input data with the read compensation value. Referring to FIG. 1, the ODC modulation method modulates the current frame data into “223” larger than “191” when the previous frame data is “127” and the current frame data is “191” and modulates the current frame data into “31” smaller than “63” when the previous frame data is “191” and the current frame data is “63” so as to improve the response characteristic of liquid crystal.
Black data insertion (BDI) method is a method for improving the response characteristic of liquid crystal by inserting a black frame between neighboring frames, thereby improve motion blurring.
To improve the 3D crosstalk, it is considered to apply the above-described methods for improving the response characteristic of liquid crystal to 3D image display devices, as shown in FIG. 2. In FIG. 2, an (n−2)th frame Fn−2 represents a left-eye data frame displaying a left-eye image, an nth frame Fn represents a right-eye data frame displaying a right-eye image, and an (n−1)th frame Fn−1 denotes a black frame displaying a black image.
However, when the BDI is applied to 3D images, a frame right before the left-eye data frame or the right-eye data frame is a black frame all the time, and thus 3D crosstalk cannot be effectively improved with conventional ODC logic and compensation values. For example, the (n−2)th frame Fn−2 and the nth frame Fn respectively corresponding to the left-eye and right-eye data frames are arranged having the (n−1)th frame Fn−1 corresponding to a black frame between them and the conventional ODC modulation method is applied to the frames, the display luminance corresponding to the nth frame Fn (having a target gray-scale value “150”) in a case A where a target gray-scale value is changed in the order of “180”, “0” and “150” by frames becomes different from the display luminance corresponding to the nth frame Fn in a case B where the target gray-scale value is changed in the order of “255”, “0” and “150” by frames. In both the cases A and B, the nth frame Fn has the same ODC value “180” in response to a variation in the target gray-scale value from “0” to “150”. However, if a time required for a display image to be changed from a specific gray-scale level to the perfect black level becomes longer than one frame due to a response time delay of liquid crystal, a right-eye (or left-eye) image is displayed in the state that the display image does not become a perfect black image. Since initial luminance Di corresponding to a rising time of liquid crystal for displaying the right-eye image in the nth frame Fn is proportional to a gray-scale difference between the (n−2)th frame Fn−2 and the (n−1)th frame Fn−1, the initial luminance Di in the case B is higher than that in the case A.
Accordingly, to remove a luminance variation, it is required to ODC-modulate data of the right-eye (or left-eye) data frame with reference to the gray-scale value of the left-eye (or right-eye) data frame. However, a black frame is inserted between neighboring data frames and then ODC modulation is performed at present, and thus it is impossible to ODC-modulate the data of the right-eye data frame with reference to the gray-scale value of the left-eye data frame.